The primary causes of inflammation in rheumatoid arthritis and Sjogren's syndrome are still unknown. These diseases are characterized by high and persistent titres of rheumatoid factors (F), antibodies that bind determinants on the Fc fragment of IgG. Transient elevations in circulating RF occur in healthy individuals with vaccination and infections. Understanding the origin of these "normal" autoantibodies and their relationship to those that arise in disease is of importance in distinguishing among models for pathogenesis. There is recent evidence that in healthy individuals RFs are enriched in a subpopulation of B lymphocytes that bear the CD5 surface marker and have distinctive requirements for stimulation. This population is expanded in fetal life, at a time when newly emerging B lymphocytes preferentially express a small subset of the encoded antibody variable region genes. Interestingly, this subset includes several genes that are known to encode autoreactive paraproteins. These observations raise the hypotheses to be explored in this project: 1) that rheumatoid factors are preferentially expressed by a subset of B lymphocytes that emerge early in fetal development 2) that disease-associated rheumatoid factors arise by pathologic activation or persistent stimulation of clones from this B cell subset. The approach will be establish immortalized rheumatoid factor secreting cell lines from four populations: newly emerging lymphocytes from fetal liver; corresponding fetal peripheral (spleen) lymphocytes, divided into CD5 positive and negative subsets; and B lymphocytes from blood and synovium of individuals with autoimmune disease. For each clone, antigen specificity will be determined by enzyme linked immunosorbant assay of binding to Fc and non-Fc epitopes, affinity will be determined by competition assays, and structure will be determined by nucleotide sequence DNA complimentary to the mRNA encoding the heavy and light chains. Data will be analyzed within each population to determine selection biases, clonal relatedness among cell lines, the identity of variable region gene segments expressed, and the contribution of junctional sequences and somatic mutations to binding specificity and affinity. Data will be analyzed between populations to define relationships in lineage and binding characteristics.